DC or direct chill casting of copper base alloys presents its own special problems in the prevention of adhesion of the molten alloy to the casting mold walls during the process. In such a casting process, the casting mold walls are normally coated or "dressed" to prevent adhesion or welding of the molten copper base alloy to the walls and to also provide lubrication between the mold walls and the emerging solidifying ingot. Various coating or dressing materials have been utilized in the prior art and such materials have usually consisted of carbonaceous powders such as graphite in a carrier of oil, grease or alcohol. The particular carrier has aided in application of the dressing material to the mold walls but has usually played an insignificant role in providing desired lubrication properties to the mold. Various other materials such as long chain organic compounds have been utilized to provide lubrication between the molten metal and the mold walls but have not achieved great commercial success.
Among other materials which have been utilized as dressing materials in direct chill casting processes, various mixtures of boron compounds such as boric oxide, metaboric acid and hydrates of the boric acids have been utilized in U.S. Pat. No. 3,342,249. These materials, according to the patent, have been substituted for such materials as graphite and have provided desirable increases in lubricating properties to the casting process. This particular patent, however, nowhere discusses the use of other particular boron compounds and in particular the boron compound utilized in the present invention.
The present invention, therefore, includes the use of a particular boron compound as lubricating material in direct chill casting processes. The present invention also contemplates the use of this same material as a covering material for the molten metal within the mold used in such a process. Typical mold covering materials have consisted of the same type of carbonaceous powders as utilized in mold dressings and other materials such as molten salts have also been used for this purpose. During the casting process, this mold covering material passes between the casting mold walls and the solidifying ingot to provide some supplementation to the lubricating properties of the mold dressing material. Use of the various prior art materials provides an inherent disadvantage in the direct chill casting of alloys which exhibit wide freezing ranges. Such alloys are those which have at least a 50.degree. C. temperature difference between the solidus temperature and the liquidus temperature. The solidification process for these types of alloys invariably leads to inverse segregation within the alloy, wherein the solute-rich low melting point liquids in the alloy flow out to the ingot surface and become deposited on the casting mold. This deposited material on the casting mold walls covers the mold dressing material and severely decreases its lubricating effect. This particular segregation in turn leads to tearing of the ingot shell and possible failure of the casting process.
It is therefore a principal object of the present invention to provide a mold dressing material which provides increased lubrication in direct chill casting processes.
It is a further object of the present invention to provide a material as aforesaid which is particularly useful for copper base alloys which have wide freezing temperature ranges.
It is yet a further object of the present invention to provide a material as aforesaid which is also useful as covering material for the molten metal within a direct chill casting process.
It is still another object of the present invention to provide a material as aforesaid which reduces tearing of the ingot shell within a direct chill casting process.
Further objects and advantages of the present invention will become apparent from a consideration of the following specification.